Method and system for optically sorting and/or manipulating carbon nanotubes

ABSTRACT

A method, apparatus, and system for optically sorting and/or manipulating carbon nanotubes by creating an optical dipole trap with a focused light source (e.g., a laser) are described in detail herein. In one representative embodiment, light from the light source may be directed onto a mixture of carbon nanotubes, the mixture including a target class of carbon nanotubes having dimensions (e.g., length and diameter) corresponding to particular electronic properties suitable for an application. By identifying a resonant condition corresponding to the target class of carbon nanotubes, and tuning the light source substantially to the resonant condition, an optical dipole trap may be created to attract carbon nanotubes of the target class to allow manipulation and/or sorting of the target class of carbon nanotubes from the mixture, or rotation of the nanotubes via rotation of a plane of polarization of the light, in an embodiment.

TECHNICAL FIELD

This disclosure relates generally to carbon nanotube technology, andmore particularly, but not exclusively, to a method, apparatus, andsystem for optically sorting and/or manipulating a target class ofcarbon nanotubes via exploitation of electronic properties correlativeto dimensions of the carbon nanotubes.

BACKGROUND INFORMATION

Carbon nanotubes have evoked considerable interest since their discoveryin the early 1990s. Potential uses include everything from transistors,digital memory, and miniature electron emitters for displays, tohydrogen gas storage devices for the next generation ofenvironmentally-friendly automobiles.

With mechanical strengths up to 100 times that of steel, carbonnanotubes are structurally seamless cylindrical tubes of graphitesheets. The basic repeating unit of the graphite sheet compriseshexagonal rings of carbon atoms, with a carbon-carbon bond length ofabout 1.42 Å. Carbon nanotubes may be capped with a fullerenehemisphere, and, depending on the method of synthesis, may comprisemulti-walled or single-walled tubes. A single-walled carbon nanotubecomprises a tube, the wall of which is only a single atom thick.Multi-walled carbon nanotubes comprise a collection of tubes, stuffedone within another in a nested configuration. A typical single-walledcarbon nanotube (“SWNT”) may have a diameter varying from about 1nanometer (“run”) to about 5 nm, and a length of up to a fewmillimeters. Generally, SWNTs are preferred over multi-walled carbonnanotubes for use in applications because they have fewer defects andare therefore stronger and more conductive than multi-walled carbonnanotubes having similar dimensions.

The structural characteristics of carbon nanotubes impart uniquephysical properties that make nanotubes suitable for a variety ofapplications. For instance, carbon nanotubes may exhibit electricalcharacteristics of metals or semiconductors, depending on the degree ofchirality or twist of the tube. Different chiral forms of carbonnanotubes are referred to as armchair and zigzag, for example. Theelectronic properties exhibited by carbon nanotubes are determined inpart by the diameter and length of the tube. Utilization of carbonnanotubes having particular electronic properties may be aided by amechanism for producing or selecting nanotubes of specified dimensions.

Existing carbon nanotube synthesis techniques include arc discharge, gasphase synthesis, laser ablation of carbon, and the chemical vapordeposition of hydrocarbons. The arc discharge method is generally notable to control the diameter or length of carbon nanotubes. Meanwhile,the gas phase synthesis method, while appropriate for mass synthesis ofcarbon nanotubes, also has difficulty in controlling the diameter andlength of the carbon nanotubes produced therefrom. While the laserablation method, and the chemical vapor deposition method generallyyield more uniform carbon nanotube products, no adequate mechanismexists for selectively synthesizing carbon nanotubes of specifieddimensions.

Known selection and/or sorting techniques for carbon nanotubes havingspecified dimensions have also proven problematic. While an atomic forcemicroscope or scanning tunneling microscope may be used to select and/orprecisely measure the geometry of individual nanotubes, sorting largenumbers of nanotubes via these instruments may be both time consumingand cumbersome. As applications for carbon nanotubes become moresophisticated, mechanisms for selectively manipulating carbon nanotubesof specified dimensions become an increasingly integral element ofdevice construction.

BRIEF DESCRIPTION OF THE VARIOUS VIEWS OF THE DRAWINGS

In the drawings, like reference numerals refer to like parts throughoutthe various views of the non-limiting and non-exhaustive embodiments ofthe present invention, and wherein:

FIG. 1 is a pictorial illustration of a carbon nanotube in accordancewith an embodiment of the present invention;

FIGS. 2A and 2B are pictorial illustrations of two example carbonnanotubes showing a difference between a pair of energy levels inaccordance with an embodiment of the present invention;

FIG. 3 is a flow diagram illustrating an embodiment of a flow of eventsin a process in accordance with an embodiment of the present invention;

FIG. 4 is a pictorial illustration of an embodiment of an apparatus formanipulating carbon nanotubes via light in accordance with an embodimentof the present invention;

FIG. 5 is a pictorial illustration of another embodiment of an apparatusfor manipulating carbon nanotubes via light in accordance with anembodiment of the present invention;

FIG. 6 is a pictorial illustration of yet another embodiment of anapparatus for manipulating carbon nanotubes via light in accordance withan embodiment of the present invention; and

FIG. 7 is a block diagram illustrating an embodiment of a system forgenerating, and manipulating and/or sorting carbon nanotubes inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Embodiments of a method, apparatus, and system for optically sortingand/or manipulating carbon nanotubes are described in detail herein. Inthe following description, numerous specific details are provided, suchas the identification of various system components, to provide athorough understanding of embodiments of the invention. One skilled inthe art will recognize, however, that embodiments of the invention canbe practiced without one or more of the specific details, or with othermethods, components, materials, etc. In still other instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring aspects of various embodiments ofthe invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearance of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

As an overview, embodiments of the invention provide a method,apparatus, and system for optically sorting and/or manipulating carbonnanotubes by creating an optical dipole trap with a focused light source(e.g., a laser). In one representative embodiment, light from the lightsource may be directed onto a mixture of carbon nanotubes, the mixtureincluding a target class of carbon nanotubes having dimensions (e.g.,length and diameter) corresponding to particular electronic propertiessuitable for an application. By identifying a resonant conditioncorresponding to the target class of carbon nanotubes, and tuning thelight source substantially to the resonant condition, an optical dipoletrap may be created to attract carbon nanotubes of the target class toallow manipulation and/or sorting of the target class of carbonnanotubes from the mixture, in an embodiment. Other features of theillustrated embodiments will be apparent to the reader from theforegoing and the appended claims, and as the detailed description anddiscussion is read in conjunction with the accompanying drawings.

With reference now to the drawings, and in particular to FIG. 1, apictorial illustration of an embodiment of a carbon nanotube 101 isshown in accordance with an embodiment of the present invention. It willbe appreciated that the figures referenced and described herein areintended for illustrative purposes only, and are not necessarily drawnto scale. The carbon nanotube 101 comprises a plurality of carbon atomsbonded together to form a cylindrical tube comprised of a lattice ofhexagons 103, and having a length 105 and a diameter 107, in anembodiment.

Carbon nanotubes (e.g., the carbon nanotube 101) have strong electronicproperties that are modulated by the length (e.g., the length 105) andthe diameter (e.g., the diameter 107) of the tube. The dimensiondependence of the electronic properties results from the one-dimensionalform of the tube, which is an approximate model of a quantum mechanicalsystem. Electrons in these cylindrical tubes are confined in the radialand circumferential directions, and can only propagate along thelongitudinal axis of the tube. The sensitivity of the electronicwavefunction to length may be illustrated, in an embodiment, by a simpleestimate for the energy level splitting (ΔE) of a tube of length L:ΔE=hv _(F)/2Lwhere h is Planck's constant and v_(F) is the Fermi velocity (8.1×10⁵m/s). For example, a nanotube of 30 nm in length may have an estimatedenergy level splitting of 0.06 eV. As will be appreciated, thedifference between electron energy levels is inversely proportional tothe length of the nanotube, with finer splitting observed for longertubes.

With reference now primarily to FIGS. 2A and 2B, pictorialrepresentations of two example carbon nanotubes 201 and 203,respectively, illustrating the dependence of energy level splitting ontube length, are shown in accordance with an embodiment of the presentinvention. FIG. 2A depicts a carbon nanotube 201 having a diameter d anda length L₁. The carbon nanotube 201 includes, in an embodiment, a firstenergy level 205 and a second energy level 207 separated by an energylevel splitting ΔE₁. FIG. 2B depicts a carbon nanotube 203 having adiameter d and a length L₂. The carbon nanotube 203 includes, in anembodiment, a first energy level 209 and a second energy level 211separated by an energy level splitting ΔE₂. As will be appreciated withreference to FIGS. 2A and 2B, L₁>L₂, and consequently, ΔE₁<ΔE₂.

The electronic properties of carbon nanotubes are also a function oftube diameter. For example, the relationship between the fundamentalenergy gap (E_(gap)) (highest occupied molecular orbital-lowestunoccupied molecular orbital) and tube diameter may be modeled, in anembodiment, by the following function:E _(gap)=2y ₀ a _(cc) /d

where y₀ is the carbon-carbon tight bonding overlap energy (2.7±0.1 eV),a_(cc) is the nearest neighbor carbon-carbon distance (0.142 nm), and dis the tube diameter.

Upon excitation by light, for example a laser, or the like, electronsare excited into higher energy states. At around 2 eV, carbon nanotubesundergo strong absorption/emission of light with a series of sharpresonant peaks determined by both the diameter and length of the tube,as described above. Because the quantized energy levels of the carbonnanotubes are dependant on the dimensions of the tube, nanotubes ofdifferent dimensions will absorb light of different wavelengths, therebyresulting in an alteration in the electron distribution within thenanotube, and a corresponding change in the molecular dipole moment. Byexploiting these electronic characteristics of the nanotubes, anembodiment of the present invention provides selective manipulationand/or sorting of the tubes via a tuned light source.

Optical dipole traps are based on the interaction between an electricfield component of light E, and an induced electric dipole moment d_(m),which is proportional to E. An interaction energy U is proportional to alocal light intensity, and may be determined, in an embodiment, by thefollowing relationship:U=−(d _(m) ·E)/2In a carbon nanotube, the molecular dipole moment may be oriented alongthe longitudinal axis of the nanotube, resulting from the electronicstanding waves formed along the tube. Thus, it will be appreciated thatby tuning a light source, such as a laser, to the specific resonantcondition corresponding to the diameter and length of a particulartarget class of carbon nanotubes (e.g., having particular dimensions),carbon nanotubes of the target class may be attracted to the region ofmaximum intensity, allowing the tubes to be manipulated and/or sorted,in an embodiment.

With reference now primarily to FIG. 3, a flow diagram illustrating anembodiment of a flow of events in a process is shown in accordance withan embodiment of the present invention. As the following discussionproceeds with regard to FIG. 3, reference is made to FIGS. 4-7 toillustrate various aspects of embodiments of the invention.

In the illustrated embodiment of FIG. 3, the process begins with thesynthesis of a mixture of carbon nanotubes (see, e.g., process block301). It will be appreciated that process block 301 is illustrated withbroken lines to indicate that this portion of the process need notnecessarily be included in all embodiments of the present invention. Forexample, in one embodiment, the synthesis of carbon nanotubes maycomprise a completely separate process, the product of which maycomprise an input to the remainder of the process illustrated in FIG. 3.In various embodiments, the mixture of carbon nanotubes of varyinglength and/or diameter may be produced by a variety of techniques knownin the art, including but not limited to carbon-arc discharge, chemicalvapor deposition, plasma assisted chemical vapor deposition, laserablation of a catalytic metal-containing graphite target, condensedphase electrolysis, or the like.

The process illustrated in FIG. 3 next proceeds, in an embodiment, toidentify a resonant condition corresponding to a target class of carbonnanotubes (see, e.g., process block 303). For example, the target classof carbon nanotubes may comprise those nanotubes having a particularlength and/or diameter that may be suitable for a particularapplication. In one embodiment, identifying the resonant condition mayinclude examining at least one carbon nanotube to identify at least onedimension corresponding to the target class of carbon nanotubes. In oneembodiment, the at least one dimension may comprise a length or adiameter, as discussed above. Examining the at least one carbon nanotubemay comprise, in an embodiment, selecting a series of nanotubes andobserving and/or measuring the dimensions of the selected series ofnanotubes via scanning tunneling microscopy, or the like. Afteridentifying a carbon nanotube having the desired dimensions (e.g.,corresponding to the target class) via examination, as described above,identifying the resonant condition corresponding to the target class ofcarbon nanotubes may also include, in an embodiment, exposing the carbonnanotube having the at least one dimension (e.g., corresponding to thetarget class) to a variable light source to identify a wavelength oflight capable to create the optical dipole trap corresponding to thetarget class of carbon nanotubes. For example, an operator may scanthrough a series of wavelengths with the light substantially focused onthe carbon nanotube until an optical dipole trap is observed (i.e., thecarbon nanotube is visibly attracted to the light).

In another embodiment, identifying the resonant condition correspondingto the target class of carbon nanotubes may include examining the atleast one carbon nanotube to identify at least one dimensioncorresponding to the target class of carbon nanotubes, as discussedabove. Following the foregoing examination, a calculation of theresonant condition may be undertaken via quantum mechanical principles,based on the dimensions of the nanotube, which would be familiar tothose having skill in the art.

With continued reference to FIG. 3, the process next proceeds, in anembodiment, to tune the light source substantially to the resonantcondition (see, e.g., process block 305) (e.g., the wavelength capableto create the optical dipole trap corresponding to the target class ofcarbon nanotubes), and to direct light, emitted by the light source,onto at least one nanotube of the target class of carbon nanotubes tocreate the optical dipole trap (see, e.g., process block 307). Directingthe light to create the optical dipole trap may then allow manipulationof the at least one nanotube via the light to rotate the at least onenanotube and/or sort the at least one nanotube from the mixture (see,e.g., process block 309), in an embodiment.

With reference now primarily to FIG. 4, an embodiment of an apparatus401 for manipulating carbon nanotubes via light is shown in accordancewith an embodiment of the present invention. In one embodiment, theapparatus 401 includes a light source 403, for example a laser, to emitlight 405 a. As will be appreciated, the wavelength of the light 405 amay be tuned to correspond substantially to the resonant condition ofthe target class of carbon nanotubes, as discussed above in conjunctionwith process block 303 (FIG. 3). The apparatus 401 may also include, inan embodiment, focusing optics 407, such as for example a lens,optically coupled to the light source 403 and configured to direct thelight 405 b onto at least one carbon nanotube of the target class ofcarbon nanotubes 413. The at least one carbon nanotube of the targetclass of carbon nanotubes 413 may, in an embodiment, comprise a portionof a mixture of carbon nanotubes 411. It will be appreciated that in oneembodiment, the focusing optics 407 may comprise an element of the lightsource 403, and need not be provided separately as illustrated in FIG.4.

In one embodiment, the apparatus 401 may also include a collector 409positioned to accumulate the target class of carbon nanotubes (e.g., thecarbon nanotube 413) in response to manipulation of the tubes with thelight. For example, in one embodiment, the apparatus 401, including thelight source 403 and the focusing optics 407, may be configured to scanthe light 405 b across the mixture of carbon nanotubes 411, as indicatedby the arrow having reference numeral 415. In this manner, carbonnanotubes of the target class (e.g., the carbon nanotube 413) may bemanipulated to, for example, sort the target class of carbon nanotubes(see, e.g., the carbon nanotube 413) from the mixture of carbonnanotubes 411 having varying lengths and/or diameters.

With reference now primarily to FIG. 5, a pictorial illustration ofanother embodiment of an apparatus 501 for manipulating carbon nanotubesvia light is shown in accordance with an embodiment of the presentinvention. In the illustrated embodiment, the apparatus 501 includes alight source 503 that may be tuned substantially to the resonantcondition corresponding to the target class of carbon nanotubes, asdiscussed above. In addition, the apparatus 501 may include a beamsplitter 505, optically coupled to the light source 503 to split light515 emitted from the light source 503 along a first optical path 517 aand a second optical path 517 b. Light propagating along the firstoptical path 517 a may be re-directed by a first mirror 509 a.

In one embodiment, the apparatus 501 may also include a firstacousto-optic modulator 507 a and second acousto-optic modulator 507 b,optically coupled to the light source 503, and positioned to control thefrequency of the light propagating along the first and second opticalpaths 517 a and 517 b, respectively. In one embodiment, the first andsecond acousto-optic modulators 507 a and 507 b may be configured toheterodyne the frequency of the light propagating along the first andsecond optical paths 517 a and 517 b, respectively. The lightpropagating along the first and second optical paths 517 a and 517 b maythen be reflected from a second mirror 509 b and a third mirror 509 c,respectively, toward focusing optics 511 a and 511 b, respectively, inan embodiment. The focusing optics 511 a and 511 b may each comprise alens, in an embodiment, and may be configured to direct the lightpropagating along the first and second optical paths 517 a and 517 bonto at least one nanotube of the target class of carbon nanotubes(e.g., within a mixture of carbon nanotubes 513) to create an opticaldipole trap capable to attract the at least one nanotube.

By sweeping the frequency of the light via one of the acousto-opticmodulators 507 a and 507 b in a phase-continuous way, an interferencepattern may be moved in one or the other direction between the focusingoptics, thereby creating a “conveyor belt” along which nanotubes of thetarget class of carbon nanotubes may be moved to separate them from themixture of carbon nanotubes 513 having varying lengths and/or diameters,in one embodiment.

In addition to sorting the target class of carbon nanotubes,manipulating the at least one carbon nanotube via the light may, in anembodiment, comprise rotating the at least one carbon nanotube viarotation of a plane of polarization of the light. Carbon nanotubes inresonance with the light (e.g., the light 405 b, FIG. 4) emitted fromthe light source (e.g., the light source 403, FIG. 4) may align alongitudinal axis of the tube parallel to the plane of polarization ofan electric field component of the light, in one embodiment.

With reference now primarily to FIG. 6, a pictorial illustration of yetanother embodiment of an apparatus 601 for manipulating carbon nanotubesvia light is shown in accordance with an embodiment of the presentinvention. In the illustrated embodiment, the apparatus 601 includes alight source 603 capable to generate light 605 a tuned to the resonantcondition of the target class of carbon nanotubes, as discussed above.In addition, the apparatus 601 may also include, in an embodiment, apolarizer 607, optically coupled to the light source 603 and capable torotate the plane of polarization of the light 605 a. The apparatus 601may also comprise, in an embodiment, focusing optics 609, such as alens, to direct the light 605 b, emitted by the light source 603 andpolarized by the polarizer 607 onto at least one nanotube 611 of thetarget class of carbon nanotubes to create an optical dipole trapcapable to attract the at least one nanotube 611. By rotating the planeof polarization of the light 605 a with the polarizer 607, the at leastone nanotube 611 may be rotated accordingly, as described above, andindicated by the arrows having reference numerals 613 a and 613 b,respectively.

With reference now primarily to FIG. 7, a block diagram illustrating anembodiment of a system 701 for generating, and manipulating and/orsorting carbon nanotubes is shown in accordance with an embodiment ofthe present invention. The system may comprise, in an embodiment, ananotube generator 703 and a sorting/manipulating apparatus 705, such asthose illustrated in FIGS. 4-6, and described herein. In one embodiment,the nanotube generator 703 may be configured to synthesize a pluralityof carbon nanotubes via any one of the techniques know in the art, or acombination thereof. The plurality of carbon nanotubes may each have alength and a diameter, and among the plurality of carbon nanotubessynthesized by the nanotube generator 703, the length and/or diameter ofthe tubes may vary according to the technique and/or parameters used toproduce the nanotubes, in an embodiment.

In one embodiment, the sorting/manipulating apparatus 705 may receivethe nanotubes produced by the nanotube generator 703, and may beconfigured to sort the plurality of carbon nanotubes according to atleast one of the length or the diameter of the tubes. For example, inone embodiment the apparatus may comprise a light source, such as thosedescribed above in conjunction with FIGS. 4-6, tuned substantially to aresonant condition corresponding to the target class of carbonnanotubes. The target class of carbon nanotubes may, in an embodiment,comprise at least a portion of the plurality of carbon nanotubesproduced by the nanotube generator 703. In addition to the light source,the sorting/manipulating apparatus 705 may also include, in anembodiment, focusing optics, such as those described above inconjunction with FIGS. 4-6, configured to direct light, emitted by thelight source, onto at least one nanotube of the target class of carbonnanotubes to create an optical dipole trap capable to attract the atleast one nanotube.

As will be appreciated, the light source of the sorting/manipulatingapparatus 705 may comprise a laser, and the focusing optics may compriseat least one lens, in an embodiment. In one embodiment, thesorting/manipulating apparatus 705 may also include a collector, such asthat described above in conjunction with FIG. 4, positioned toaccumulate the target class of carbon nanotubes in response tomanipulation with the light. In still another embodiment, thesorting/manipulating apparatus 705 may include a polarizer, opticallycoupled to the light source, to rotate a plane of polarization of thelight to rotate the at least one nanotube, the at least one nanotubehaving a longitudinal axis aligned parallel to the plane ofpolarization.

In one embodiment, the sorting/manipulating apparatus 705 may beconfigured to scan the light, emitted by the light source, across amixture of carbon nanotubes (e.g. the plurality of carbon nanotubesproduced by the nanotube generator 703) to create an optical dipole trapto attract nanotubes of the target class to allow sorting and/or othermanipulation. The mixture may, in an embodiment, comprise a dispersionof carbon nanotubes in a medium, as will be familiar to those skilled inthe art.

While the invention is described and illustrated here in the context ofa limited number of embodiments, the invention may be embodied in manyforms without departing from the spirit of the essential characteristicsof the invention. The illustrated and described embodiments, includingwhat is described in the abstract of the disclosure, are therefore to beconsidered in all respects as illustrative and not restrictive. Thescope of the invention is indicated by the appended claims rather thanby the foregoing description, and all changes which come within themeaning and range of equivalency of the claims are intended to beembraced therein.

1-18. (canceled)
 19. A system, comprising: a carbon nanotube generatorconfigured to synthesize a plurality of carbon nanotubes, each of theplurality of carbon nanotubes having a length and a diameter; and anapparatus configured to sort the plurality of carbon nanotubes accordingto at least one of the length or the diameter, the apparatus comprising:a light source tuned substantially to a resonant condition correspondingto a target class of carbon nanotubes, the target class of carbonnanotubes comprising at least a portion of the plurality of carbonnanotubes; and focusing optics configured to direct light, emitted bythe light source, onto at least one nanotube of the target class ofcarbon nanotubes to create an optical dipole trap, the optical dipoletrap capable to attract the at least one nanotube.
 20. The system ofclaim 19, wherein the apparatus is configured to scan the light across amixture of carbon nanotubes, the mixture of carbon nanotubes includingthe target class of carbon nanotubes.
 21. The system of claim 19,wherein the light source comprises a laser.
 22. The system of claim 19,wherein the focusing optics include at least one lens.
 23. The system ofclaim 19, wherein the apparatus further comprises a collector positionedto accumulate the target class of carbon nanotubes in response tomanipulation by the light.
 24. The system of claim 19, wherein theapparatus further comprises a polarizer, optically coupled to the lightsource.
 25. The system of claim 24, wherein the polarizer is configuredto rotate a plan of polarization of the light to rotate the at least onenanotube, the at least one nanotube having a longitudinal axis alignedparallel to the plane of polarization.
 26. A method, comprising:synthesizing a plurality of carbon nanotubes to generate a mixture ofcarbon nanotubes, each of the plurality of carbon nanotubes having alength and a diameter; and sorting the mixture of carbon nanotubesaccording to at least one of the length or the diameter, sorting themixture, comprising: identifying a resonant condition corresponding to atarget class of carbon nanotubes; tuning a light source substantially tothe resonant condition; directing light, emitted by the light source,onto at least one nanotube of the target class of carbon nanotubes tocreate an optical dipole trap; and manipulating the at least onenanotube via the light.
 27. The method of claim 26, wherein identifyingthe resonant condition includes: examining at least one carbon nanotubeto identify at least one of the length or the diameter corresponding tothe target class of carbon nanotubes; and exposing a carbon nanotubehaving the at least one dimension to a variable light source to identifya wavelength of light capable to create the optical dipole trapcorresponding to the target class of carbon nanotubes.
 28. The method ofclaim 26, wherein the light source comprises a laser.
 29. The method ofclaim 26, wherein directing light, emitted by the light source, onto theat last one nanotube includes scanning the light across the mixture ofcarbon nanotubes, the target class of carbon nanotubes comprising aportion of the mixture.
 30. The method of claim 29, wherein the mixtureof carbon nanotubes comprises a dispersion of carbon nanotubes in amedium.